1. Field of the Invention
The invention relates to web handling systems and, more particularly, relates to a system for threading endless webs through a machine such as a dryer.
2. Description of the Related Art
In a variety of applications, a continuous web formed from paper, film, foil, or other materials is fed through a machine such as a dryer in which an operation such as drying is performed on the web. The web is typically pulled through the dryer or other machine during operation of the machine by pulleys or other devices located outside cf the machine. However, during start up of the machine, the web must be threaded through the machine by an internal threader assembly.
The typical threader assembly comprises a belt or a chain which grasps an edge of the web and which pulls the web through the machine to thread the web. One such machine is disclosed in U.S. Pat. No. 3,399,465, which issued to Lanne et al. on Sep. 3, 1968. The web threader disclosed in Lanne et al. includes an endless conveyor composed of two substantially planer belts which are united at their outer lateral edges and which are bent over one another as they travel through a dryer to enclose the leading end of the web material therebetween. The two parts are separated at the outlet of the dryer to release the web.
The web threader disclosed by Lanne et al., as well as other web threaders which were heretofore available, suffer from several drawbacks and disadvantages. First, rubber or canvas belt-type threaders of the type disclosed in Lanne et al. are difficult to clean, cannot withstand high temperatures which may be present in some dryers, and may be susceptible to attack by solvents present in many devices in which threaders are employed. Such solvents are often present in high concentrations in dryers which handle solvent-laden paper. Moreover, the overlapping belt-type threader disclosed by Lanne et al. also is necessarily relatively thick and thus cannot be used on equipment requiring very tight sealing, such as inert atmosphere enclosures in which an inert gas such as nitrogen is induced to reduce oxygen levels, thus inhibiting combustion. The typical threader also obstructs the viewing area located above the web, thereby inhibiting visual monitoring of the threader operation. Because the belt-type threader disclosed by Lanne et al. necessarily touches the top of the web, it cannot be used to thread webs having a wet coating on the entire widths of their upper or lower faces without smudging or smearing the coating.
Some of the disadvantages associated with belt-type threaders can be avoided through the use of chain-type threaders which grasp the edges of the webs. For instance, chain-type threaders are less susceptible to corrosion from solvents and are more suitable for higher temperature applications. However, such threaders have limited flexibility and are also relatively bulky and difficult to clean, thus limiting their suitability for use in equipment requiring tight sealing or in configurations requiring the conveying of the webs through complex paths. These chains are also relatively heavy and thus can span only relatively short distances without supports. Chain-type conveyors also usually require lubrication and thus cannot be used in applications requiring extreme cleanliness.